In air-fuel ratio control of a fuel mixture supplied to an engine, deviation of an air-fuel ratio during engine acceleration/deceleration from a target value is often related to so-called wall flow of fuel. Wall flow refers to a phenomenon wherein, for example, fuel injected from a fuel injection nozzle adheres to an engine intake valve or intake port, and flows down walls into a cylinder as a liquid. Due to the fact that the flowrate of this wall flow varies depending on the acceleration/deceleration of the engine, the air-fuel ratio of the fuel mixture also varies.
In air-fuel ratio control, the fuel supply amount is usually corrected by considering oversupply or undersupply of fuel due to wall flow as a transient correction amount.
For example, an equilibrium adhesion mount Mfh and quantity proportion Kmf are first determined according to an engine load, engine speed Ne and cooling water temperature Tw, and an adhesion rate Vmf is found from a mathematical expression using these values.
The equilibrium adhesion amount Mfh is a fuel adhesion amount in a steady state determined by the engine speed and temperature of a fuel adhering part. The quantity proportion Kmf is a factor indicating the extent to which a difference (Mfh-Mf) between the equilibrium adhesion amount Mfh and an adhesion amount Mf at present can be reflected in a correction of fuel injection amount. The adhesion rate Vmf is an adhesion amount per unit fuel injection period (per unit injection), and a basic fuel injection amount Tp is corrected by this adhesion rate Vmf.
However, in the case of an engine where the fuel injection nozzle injects fuel towards the intake valve, a large error appears in the air-fuel ratio when the equilibria adhesion amount Mfh and quantity proportion Kmf are computed from the cooling water temperature Tw, and this is especially true immediately after a cold start. In this case, the wall flow fuel amount is affected by the temperature of the intake valve on the surface of which the wall flow is flowing, and the temperature difference between the valve temperature and cooling water temperature Tw leads to an error in the estimate of wall flow.
In this context, Tokkai Hei 1-305142 published by the Japanese Patent Office in 1989 discloses a method wherein the valve temperature is first estimated, and the valve temperature is used instead of the cooling water temperature Tw for computing Mfh and Kmf. Immediately after start-up, the valve temperature is effectively the same as the cooling water temperature Tw, and it levels off to a temperature that is higher than Tw by a constant value (e.g. approx. 80.degree. C.). Also, the variation of the valve temperature is a first order delay depending on a time constant determined by the engine air intake volume. A predicted valve temperature Tf can therefore be found from the following equation: EQU Tf=Th.multidot.SPTF+Tf.sub.-1 .multidot.(1-SPTF) (1)
wherein, Tf.sub.-1 is the value of Tf on the immediately preceding occasion.
An equilibrium intake valve temperature Th and delay time constant SPTF are first determined using the engine load and speed as parameters.
In Tokkai Hei 3-134237 published by the Japanese Patent Office in 1991, a wall flow correction temperature Twf which converges toward the cooling water temperature Tw with a first order delay, is used instead of the cooling water temperature Tw.
According to the aforesaid embodiment, the data used for calculating Mfh and Kmf correspond to the case where the valve temperature has levelled off to the temperature which is higher by a predetermined amount, i.e. to an equilibrium temperature state, when the cooling water temperature Tw is constant. Consequently, in a non-equilibrium temperature state, Mfh and Kmf found using this data contain an appreciable error. As a result, a transient correction amount of a non-equilibrium temperature state may still contain an appreciable error, since calculations are performed based on Mfh, Kmf which have large errors even when a non-equilibrium temperature state is simulated using the wall flow correction temperature Twf instead of the cooling water temperature Tw.
More specifically, in the aforesaid methods, the equilibrium temperature state wherein Twf=40.degree. C. (cooling water temperature Tw=40.degree. C.) and the non-equilibrium temperature state wherein Twf=40.degree. C. (cooling water temperature Tw is not 40.degree. C.) are considered as being the same. This tends to cause errors in the a-fuel ratio immediately after start-up when non-equilibrium temperature conditions prevail continuously.